This invention relates to optical communications systems and, more particularly, a gain control system for optical amplifiers for use in such systems.
Large capacity optical transmission systems typically combine high speed signals on a signal fiber by means of Wavelength Division Multiplexing (WDM) to fill the available bandwidth. In these WDM optical transmission systems, in general, rare-earth doped fiber optical amplifiers (such as Erbium or Erbium-Ytterbium doped) are used to compensate for the fiber link and splitting losses.
Doped fiber amplifiers can provide of low noise and high gain, but they do suffer from various distortions.
As one example, gain tilt is the measure of the slope of the wavelength dependent gain of a fiber amplifier. It arises because the gain of an Erbium-doped amplifier is inherently dependent upon the absorption and emission wavelength spectrum of the Erbium ions in the fiber. One effect of gain tilt on an optical signal being amplified is an amplitude modulation of the optical signal (an unwanted distortion) by any wavelength variation in the optical signal. As a result of gain tilt, an amplifier will not provide uniform gain for different channels of a WDM system. It is known to provide dielectric filtering elements with the amplifier, and these can reduce gain tilt for a specific gain setting of the amplifier.
Gain transients, where step changes in the amplifier gain are caused by variations in input signals, are also a major problem for WDM optical systems. Gain transients occur because channels are added or dropped either due to network reconfiguration or failures. Adding channels can depress the power of the present channels below the receiver sensitivity. Dropping channels can give rise to error events in the surviving channels because the power of the surviving channels can surpass the thresholds for non-linear effects. The error bursts in the surviving or present channels as a result of these power transients are unacceptable to service providers.
Some of these effects can be eliminated if the amplifier gain, and hence gain spectrum, is controlled independently of input signal level. In this way, a constant gain can be maintained regardless of the number of channels present at the input. This requires rapid gain control to respond to channel adding and dropping at the input, without giving rise to large or prolonged gain transient effects. Known systems for implementing independent amplifier gain control use automatic gain control (AGC) in the form of opto-electronic or all optical feedback loops.
AGC schemes may use feed-forward or feedback loops, or a combination of these, in order to derive control signals from measures of input and output powers so as to increase the amplifier pump power when more output power is required. Typically, an error signal is generated which represents the difference between a desired output power level and the actual output power level, with the desired output power level being calculated from the measured input power taking into consideration the desired constant gain of the amplifier.
The calculation of the desired output power essentially involves multiplying the input power by the desired gain and adding a compensation factor for amplified stimulated emission (ASE). This ASE contributes to the measured output power of the amplifier, but is not representative of the gain imparted onto the optical signal. Clearly any error in the ASE compensation factor gives rise to errors in the desired output power, so that the gain control loop fails to achieve the desired signal amplification.
The invention is based on the recognition that the ASE component of the output power is temperature dependent, and furthermore is temperature dependent in a predictable manner.
According to the invention, there is provided an optical amplifier comprising:
a doped fiber;
a pump source for providing pump light to the fiber;
a power measurement circuit for measuring the input and output power of the amplifier;
a driver circuit for controlling the pump source, the power measurement circuit and the driver circuit comprising an amplifier gain control loop, the driver circuit estimating a desired output power based on the measured input power and a desired gain to be provided by the amplifier,
wherein a temperature measurement device is provided, and wherein the estimation of the desired output power takes into consideration the temperature provided by the temperature measurement device.
The invention provides a gain control scheme in which a measured input power is used to calculate a desired output power taking into consideration the gain setting of the amplifier and the temperature conditions. This enables a flatter gain response to be achieved over varying input power, which enables an increase in the dynamic range of the amplifier and thereby a possible increase in system capacity of a WDM system using the amplifier.
Preferably, the doped fibre comprises an Erbium doped optical fiber and the pump source comprises a laser diode, and wherein the driver circuit provides a laser diode drive current.
The input and output power are preferably supplied to a processor which generates control signals for controlling the driver circuit. In particular, an error signal may be derived from the estimated output power and the measured output power, which is processed using a proportional and integral controller, and the measured input power is processed using a differential controller.
The amplifier is for use in an optical transmission system comprising an optical transmitter and an optical receiver, with an optical fiber link between transmitter and the receiver comprising one or more amplifiers of the invention.
The invention also provides a method of controlling an optical amplifier comprising a doped fiber and a pump source for providing pump light to the fiber, the method comprising:
measuring the power at the input and output of the amplifier;
estimating a desired output power based on the measured input power and a desired gain to be provided by the amplifier;
controlling the pump source to achieve the desired gain by processing a signal derived from the measured output power and the estimated desired output power,
wherein the estimation of the desired output power takes into consideration a temperature provided by a temperature measurement device.